The invention relates to a method for manufacturing and using a screen for laser front projection of one or more laser wavelengths, which screen selectively back-scatters the incident narrow-band laser radiation in a previously determined solid angle, but simultaneously considerably absorbs the disturbing broad-band ambient light.
Shortly after powerful lasers operating in the visible region of the spectrum became available in the 1960s, initial attempts at laser projection of images on walls and screens also took place. In this connection see for example C. E. Baker, xe2x80x9cLaser Display Technology,xe2x80x9d IEEE Spectrum, December 1968, pp. 39-50. At the 1970 World""s Fair in Osaka, Japan, Hitachi presented a laser color television production on a projection screen measuring approximately three meters by four meters, as described for example in the article xe2x80x9cLarge Color Television Projection by Laserxe2x80x9d in the journal Funkschau, Volume 4, 1970.
Although these first laser projections achieved significant quality in terms of image brightness, resolution, and color fidelity, the technical investment and cost remained very high, especially because of the inefficient argon and krypton lasers used. These lasers had an electrical-optical conversion efficiency of less than 0.1 percent. Consequently, laser projection remained limited to scattered applications such as light shows, special large-image displays in the military area, or for flight simulation in pilot training.
In recent years, the technical requirements for producing laser displays have considerably improved, as for example in the article by C. Deter, xe2x80x9cLaser display technologyxe2x80x94where are we?xe2x80x9d, in the journal Physikalische Blxc3xa4tter, 52, (1996), No. 11, p. 1129. Today, much more efficient and economical diode-excited solid-state lasers or fiber lasers, and in the future laser diodes as well, with an electrical-optical efficiency of 10-30 percent can be used for image projection in the monochromatic laser colors of red, green, and blue (RGB). To produce an image by scanning, deflection units built using less expensive silicon technology will be available.
It can be expected that in the case of the projection methods already introduced, which operate similarly to slide projection using the light valve principle, liquid crystals or micro-mirror matrices that can be modulated (as well as in the future multicolored laser light) will be used.
An image can be generated on the screen basically in two ways, by front projection or rear projection. In the former case, the image is cast on the surface of the screen that is also viewed. In this case, this screen must back-scatter the incident light as much as possible. In the second case, the image is projected on the opposite side of the screen. The screen must then allow the light to pass through as much as possible but, at the same time, must forward scatter over a larger angle. The invention relates exclusively to the first method of front projection and reproduction of the back-scattered light, and the design and manufacture of such a screen for laser projection.
One of the principal advantages of laser image projection is the high luminance that can be produced with lasers in contrast to other sources, even on large projection screens. The photometric luminance (cd/m2) in the visible portion of the spectrum corresponds radiometrically to the radiance of a light-emitting surface, in other words, the power radiated in the solid angle by a unit area (w/sr m2). The radiance of conventional thermal radiation sources such as incandescent lamps, arc lamps, or gas discharge lamps has a physical limit set exclusively by its color temperature, in other words the color-equivalent temperature of the black body. An increase in its radiance by raising the temperature is only possible with a simultaneous shift of the color toward shorter wavelengths and the associated change in psychophysical color (blue tinge). Since the product of the projection angle and the area of the image exit opening are fixed for each system, it basically limits the light flux that can be used for projecting thermal radiators.
This product, the light emittance of the optical system, is very low in the case of lasers and permits a very sharp bundling of the radiation by the exit opening of the projector. As a result of the spectrally selective light amplification used in lasers, in contrast to thermal radiators, with a correspondingly strong excitation, the output power and hence the radiance that can be achieved in the useful beam are theoretically unlimited.
Since almost all colors in the color triangle can be produced by adding the narrow-band laser lines of the RGB colors, lasers are especially well suited as light sources for projection systems because of the above-mentioned advantages. This is true for both known projection methods: both for those that generate the image serially by point by point scanning of the screen with a bundled and modulated beam, and those in which all of the image points in an image matrix are illuminated simultaneously and projected on the screen.
However, a high luminance of the image is not sufficient by itself to produce images on the screen with high contrast and good color quality, since, because of the background brightness of the surrounding area, the brightness of the darkest point on the screen cannot go below this level and the color perceived by the eye is formed by the sum of the background color of the screen influenced by the environment and the color of the projection at the moment.
The contrast that can be achieved during image reproduction as a ratio of the maximum and minimum luminances therefore depends on the ambient illumination. In dark rooms, good projectors offer a perceived contrast of more than 200:1. In practical operation in bright rooms, the values are about 40:1. This is the result of the increase in the black value by the ambient light and the distortion of the gray level graduation of the eye at higher luminances (Wber-Fechner Law). The perceived psychophysical color and color contrasts are simultaneously altered in the same way by the ambient light.
These problems of image reproduction have been known for a long time in television technology, see for example xe2x80x9cFernsehtechnik, Hxc3xcttig Verlag, Heidelberg, in 1988 and are eliminated by a so-called xe2x80x9cblack matrixxe2x80x9d. The black matrix is supported inside the image tube on optically unused parts of the image tube and absorbs the ambient light.
Any increase in the luminance of the screen in front projection using cathode ray tubes, liquid crystal or micro-mirror matrices is achieved by using a special optical coating on the screen that back-scatters the light in a narrow angle and/or by concave curvature of the surface of the screen. At the same time the contrast is improved, since the ambient light striking this screen laterally is no longer diverted toward the viewer.
Several experiments are also known involving projection of broad-band light on special holographic screens; see for example R. L. Shie, C. W. Chan, and J. M. Lerner, xe2x80x9cSurface relief holography for use in display screens,xe2x80x9d SPIE, Vol. 2407, pp. 177-184. Here, the light scattering of a surface relief hologram is used for image reproduction and the scattering characteristic is adjusted by a special design of the hologram.
Despite these measures, however, no one has yet succeeded in achieving approximately the same quality of brightness, contrast, and color reproduction of a front projection in bright rooms as with direct display on the screen of a cathode ray tube.
On the other hand, the basic problems with image tubes persist: their volume and weight. Large images therefore can only be produced separately on image tubes arranged side by side, with the known disadvantages of disturbing spaces in between, nonuniform brightness, and unbalanced colors.
The goal of the invention is to provide, preferably for laser projection, a projection screen which back-scatters the narrow-band laser light in one or more colors with high efficiency in a specific solid angle, but largely absorbs the broadband ambient light and thus avoids the disadvantages of known screens.
With this projection screen according to the invention, for the first time large images can stand out from the bright surroundings even with normal ambient illumination in bright rooms or in daylight. Secondly, the contrast originally available in the device is provided to the viewer via the screen. Thirdly, color distortion or shift by undesired addition of the image colors to those of the environment is minimized.
This goal is achieved according to the invention by a holographic screen. The object of this holographic screen is preferably an adapted white screen on which all the laser projection wavelengths used in the hologram preferably shine. During recording, care is taken to ensure that the screen is illuminated with the object beam so that its back-scattering characteristic is the same as desired later in the application. An expanded beam bundle is employed as the reference beam in the holographic recording, and takes its departure from a suitable location like the later projection beam. For reproduction, either a surface beam as in the recording or a beam that scans point by point can be used in a hologram.
In order to exhibit the required angle and wavelength selectivity, the holographic screen must have the properties of a volume hologram. This is advantageously achieved by recording a reflection hologram in one or more xe2x80x9cthickxe2x80x9d recording layers (approximately 5 to 30 xcexcm). Volume grating structures result during the recording and processing of the hologram as an image of the screen independently of one another at the various wavelengths used. Under the so-called Bragg interference condition of the grating structure, which is fulfilled each time only for one wavelength and one illumination angle, light is reflected backward. When the hologram is viewed, a bright reflected image of the screen appears, with its original scatter characteristic. This is repeated for other discrete wavelengths with their associated grating structures within the same layer or other layers to form a superimposed total image which, when the color tuning is correct, shows the image of the original white screen. Light of other wavelengths as well as broadband light, because it fails to comply with the Bragg conditions, is allowed to pass through largely unattenuated if it does not strike exactly from the direction of laser projection.
If the hologram is mounted on a black layer in optical contact, the holographic screen appears dark or black under surface illumination with broad-band ambient light. On the other hand, if the surface of the holographic screen is scanned by laser beams from the correct illumination direction, in other words from the location of the previous fixed reference beam, the original image of the screen is again built up serially. If the individual laser beams are modulated with image data, the viewer sees in the holographic recording light the image as if it were appearing on the original screen but only with the improvements according to the invention described above.
Even when the light valve principle is used for image modulation, in other words the surface illumination and projection of an image matrix, the screen recorded in this fashion may be used. Care must be taken however that the same laser lines are used during recording and reproduction, and that projection takes place from the same location as the reference beam. To secure good image quality, care must be taken to ensure that the distance of the image source from the screen approximately corresponds to a point source, which is always the case at conventional projection distances.
The production of a holographic projection screen according to the invention is described above as an example. However, it can be performed in a number of different ways and in different steps that are known and understandable to the individual skilled in the art. The invention will be described in greater detail below with reference to the embodiments shown partially schematically in the figures.